[Bug target/103069] cmpxchg isn't optimized

["wwwhhhyyy333 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103069

--- Comment #11 from Hongyu Wang <wwwhhhyyy333 at gmail dot com> ---


For the case with atomic_compare_exchange_weak_release, it can be expanded as

loop: mov    %eax,%r8d
      and    $0xfff80000,%r8d
      mov    (%r8),%rsi <--- load lock first
      cmp    %rsi,%rax <--- compare with expected input
      jne    .L2 <--- lock ne expected
      lock cmpxchg %r8d,(%rdi)
      mov    %rsi,%rax <--- perform the behavior of failed cmpxchg
      jne    loop

But this is not suitable for atomic_compare_exchange_strong, as the document
said

Unlike atomic_compare_exchange_weak, this strong version is required to always
return true when expected indeed compares equal to the contained object, not
allowing spurious failures. If we expand cmpxchg as above, it would result in
spurious failure since the load is not atomic. 

So for

 do
   pd->nextevent = __nptl_last_event;
 while (atomic_compare_and_exchange_bool_acq (&__nptl_last_event,
                                              pd, pd->nextevent));

who invokes atomic_compare_exchange_strong we may not simply adjust the
expander. It is better to know the call is in loop condition and relax it
accordingly.